It is now common to fully implant a cardiac pacer within the human body in order to stimulate the heart, either on demand or synchronously, in order to ensure the continued periodic beating of the heart. Such units can often operate for years without need of attention.
In order to increase the reliability of heart pacers, it has been proposed previously to replace the RC oscillator of the older cardiac pacers with a more stable, mechanically controlled oscillator in order to provide a fundamental frequency source whose output frequency is substantially independent of temperature, power supply voltage, external fields, etc. In this connection, digital circuitry, in the form of microcircuits, have been developed and used in order to fully make use of the capabilities and stability of a mechanically controlled oscillator frequency source. In addition, other circuitry has been designed to increase the reliability of the cardiac pacer.
However, there is always the danger, in a fully implanted cardiac pacer, that one or more components will fail or wear out. While it is uncommon for the semiconductor circuit elements to fail in use, other components, such as the battery, have a normal life span of several years or so. While one way of maintaining proper operation of the pacer is to replace the battery periodically, the battery may fail or age prematurely. Thus, it may be vitally important that the condition of the battery or other component be made available or communicated to an outside observer. In this way, premature failure or aging of a component can be detected and the component replaced before the pacer fails to operate properly.
It is therefore a primary object of this invention to provide circuitry whereby a change of state of a condition being monitored is detected and communicated outside of the pacer. Other objects of the invention include the provision of circuitry in a cardiac pacer which is reliable, inexpensive, and simple and which contributes to the reliability of a fully implanted cardiac pacer.